Small -- very small -- river is most agreeable (8)
I believe the answer is:
sweetest
'most agreeable' is the definition.
(I've seen this before)
'small very small river' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'very small' becomes 'wee' ('wee' is a Scottish word for small).
'river' becomes 'test' (the River Test).
's'+'wee'+'test'='SWEETEST'
'is' acts as a link.
(Other definitions for sweetest that I've seen before include "Superlative winning" , "most agreeable" , "most attractive" , "Most appealing" , "Most saccharine or sugary" .)